An art form primarily associated with combat. Like dance, combat can be formalized from an instinctive act of belligerants into a set of rituals, techniques, and forms.
It is worth noting that the Japanese consider all art forms associated with the samurai class (collectively bushido) to be martial arts, despite many of them, e.g. go-do, or the tea ceremony, having a dubious connection to combat at best.
A fairly comprehensive list of specific martial arts can be found in that node.